Incommensurate magnetic modulation in K-rich cryptomelane, K$_x$Mn$_8$O$_{16}$ ($x\approx1.45$)

TL;DR

This study investigates the complex magnetic behavior of K-rich cryptomelane, revealing incommensurate and helical magnetic structures through neutron diffraction and magnetic measurements, with transitions at 184K, 54.5K, and 24K.

Contribution

The paper provides the first detailed characterization of incommensurate magnetic modulation in K-rich cryptomelane, combining neutron diffraction and magnetic analysis to elucidate its magnetic phases.

Findings

Incommensurate magnetic structure with a wave vector of 0.36902(15)

Multiple magnetic transitions at 184K, 54.5K, and 24K

Evidence of a helical magnetic component in the low-temperature phase

Abstract

Cryptomelane is a hollandite-like material consisting of K$^+$ cations in an $\alpha$-MnO$_2$ tunnel-like crystallographic motif. A sample with stoichiometry K$_{1.448(3)}$Mn$_8$O$_{16}$ has been synthesised and its magnetic properties investigated using variable-temperature magnetic susceptibility, heat capacity, and neutron powder diffraction. Three distinct transitions at $T_1=184$\,K, $T_2=54.5$\,K, and $T_3=24$\,K are observed. At $T_1$ there is a subtle tetragonal$\rightarrow$monoclinic transition associated with Mn$^{3+}$/Mn$^{4+}$ ordering, and a set of non-magnetic superstructure peaks emerge; these could not be indexed definitively and are indicative of an ordering that is incommensurate with the unit cell. Magnetic Bragg peaks emerge below $T_2=54.5$\,K, and their positions indicate an incommensurate modulated magnetic structure. The model consistent with the data is a dual-$\vec{k}_\mathrm{mag}$ structure with a ferromagnetic $|\vec{k}_\mathrm{mag}|=0$ component and an incommensurate $\vec{k}_\mathrm{mag}$ parallel to the $\alpha$-MnO$_2$ tunnels [$|\vec{k}_\mathrm{mag}|=0.36902(15)$], with the latter most likely to be helical. The period of oscillation of the helical component is in line with predictions based on a Heisenberg spin Hamiltonian [Mandal \textit{et al}. Phys. Rev. B 90, 104420 (2014)]. Below $T_3=24$\,K, there is a magnetic transition, which gives rise to a different set of magnetic Bragg peaks indicative of a highly complex magnetic structure.